Piezo electric oscillator



Patented Oct. ,1 3, 1931 RJi'YIvZONDil. EEISZNG, .GF MILLBURN, ASSIGNOR TO BELL TELEPHONE LABORAT'GRIES, INCCRPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK PIEZQ ELECTELIC CSCILLATGR Application filed May 1, 1929. Serial No. 359,571.

This invention relates to control of operation of electric circuits and especially to frequency control of oscillators by piezo-elee tric crystals, the application being a continua ion in part of my application, Serial No. M8225, iiled July 25, 1927.

An object of theinvention is to increase the amount of power that can be conveniently controlled by a piezo-electric body.

lVhen a piezoelectric crystal is used tocontrol a circuitthe control may be dependent upon temperature, since variations in the temperature of the crystal body cause it to change its natural frequency of vibration. Another object'of the'i'nvention is to reduce undesired I variations due to temperature changes as, for example, to reduce variations in the frequency of a crystal controlled electric soace discharge oscillator due to temperature variations of the crystal device.

in one embodiment the invention comprises the use of a plurality ofcrystal bodies connected in series to control an electric space discharge tube oscillator so that the. voltage across each crystal body is but a portion of the total voltage. Asa result, the plate voltage of the oscillator tube and consequently thepower output of the oscillator may be greatly increased without danger to any of the crystals. p

employing a plurality of piezoelectric crystal bodies for controlling a circuit and cutting part of them so that they have tem perature coefiicients of frequency opposite in sign to those of the remainder, variations due to changes in the temperature of the crystal control device, for instance oscillator frequency variations in the case of the oscillator just mentioned, can be, reduced. TheeXpression temperature coeflicient of frequency as applied to a crystal body refersto the rate of change ofthe natural frequency of. vibration of the body with its change of temperature. The rate and therefore the coeflicient is positive or negative according to whether the frequency increases T or decreases with increase of temperature. I

If a piezo-electric crystal body or element is cut from a natural crystal so that its electrode faces are in planes parallel to the optical axis and an electrical axis of the. crystal,

it will ordinarily have a positive temperature coefiicient of frequency. If, it is cut so that its electrode faces are in planes parallel to the optical axis and perpendicular to an electrical axis of the crystal, it will have a negative temperature coeificient of frcquency. However, the positlve temperature.coeflicient n the first case Wlll ordinarily be several tunes as large as the negative temperature coefficient in the second case.

Another feature of he invention isan alternative arrangement in which only two crystal elements are used and these have opposite temperature coefiicients, and the positive cceflicient crystal element has connected across it a condenser, which may be varied so -that the reactance change ofthe negative coefficient crystal element with changes in temperature is sufficiently large in proportion to the reactance change of the positive coeflicient crystal element with changes in providezero coefiicient cont-r01.

Another feature of this invention is a second alternative arrangement similar to that just mentioned in which a resistance is connected in shunt to the positive coeflicient temperature, at the operating frequency to crystal'element insteadof a condenser, to accomplish the same result.

In order to control the oscillations of the system at the desired frequency a condenser may be connected between the grid and plate of the space discharge tube to provide additional capacity at this point in the circuit. It will then also-serve to increase the feed back to the crystals.

In the drawingsi Fig. 1 is a diagram of an oscillator having separate series connected control crystal elements in accordance with the invention;

Fig. 2 is an alternative arrangement for a plurality of crystal elements connected in the circuit to the left of the broken line in Fig. l;

3 is a second alternative arrangement for that por tion of the circuit to the left of the broken line in Fi 1 in which only two piezo-el-ectric crystal elements are used;

l is a third alternative arrangement in which again only two crystal elements are used;

Fig. 5 is a graph showing the reactance curves of a positive and a negative temperature cectiicient crystal element, cut to vibrate at the same frequency; and

Fig. '6 shows the changes in the curves of a positive coeiiicient crystal element effected by connecting a condenser, ant. by connecting a resistance, in shunt thereto.

The vacuum tube oscillator in Fig. 1 comprises a three-electrode space discharge tube O with an input circuit including four piezoelectric crystal elements 1 (hereinafter to be refcrreu to crystals) connected in series, and with tuned plate circuit 2. It is not necessary that this plate circuit be tuned to a particular frequency and in fact a suitable coil alone might replace the condenser and coil comprised in the tuned circuit. Preferably, the crystals have the same fundamental natural frequency of vibration or very nearly the same frequency, althou h this is not necessary. For e ;ainple,-that or one may be a subunultiple, such as or etc, of that of'tl'ieothers. The operating frequency is not a natural, that is, a resonant frequency, but is near it, and the combined reactance of all of the crystals is an inductance say L A source of negative grid potential 4, a condenser and grid leak resistance 5, and a choke coil 6 are shown connected acro-ss't-he grid and filament for supplying biasing potential to the grid. Any of these grid biasing elements, or combinations of them, may be used to the grid properly. When the choke coil is not used, the by-pass condenser across the resistance also is not used.

The plate circuit 2, coupled by coil 3 to a load circuit Z of any desired type, operates at such a frequency as to have an inductive reactance, say L L and L together with the grid-plate capacity form a tuned circuit. Thus the oscillator is of the well-knowfn Hartley type. Nhen the tube is delivering its maximum power there will be a certain alternating plate voltage E between the plate and filament. his voltage will occur when delivering this amount of-power'regardless of the magnitudes of L and L With the given L and grid-plate capacity, the grid inductance L will also be determined. A supplemental condenser 7 may be connected across the plate and grid of the tube to provide additional capacity, if desired. Under maximum power conditions there is usually obtained a fixed E or alternating grid-filament voltage. U instead of the series connected usual, this voltage lfl occurring across the hood of a few hundred volts. Connecting ll these crystals are identical only a quarter voltage on the oscillator tube before a dan- Consequently,

controlled. oscillator.

ten'iperature changes, part of the crystals may crystals. For example, two of the crystals their principal face, perpendicular to a natuhave temperature coefficients opposite in si 'n times have opposite temperature coeilicients.

pend only on the signs of their temperature ative magnitudes. The algebraic sum of all of 'a crystalhaving a positive temperature of a crystal having a negative temperature crystals shown in Fig. 1 have been replaced positive temperature coefiicient, connected in rangcment shown in l.

l O u Q as an oscillator, the frequency of its vibration Thus, for example, the oscillations generated crystals 1 a single crystal were employed as crystal would give trouble when it reached certain values, ordinarily in the neighbor- A-I, -5 1.3 ..l L 4: the ioui ciysiais i in series is a method oi reducing the voltage across each crystal used.

of the voltage occurs across each, and the Value to which it is possible to raise the plate gerous voltage across each crystal is reached, is it 'fl'Oj'illfitltfily four times as high as if a single crystal were used.

much more power can be secured from the To reduce undesired variations in the frequency of the controlled oscillator due to h vetemperature coeflicients of frequency opposite in sign to those of the remainder of the in Fig. 1 may be quartz crystals cut with their electrode laces, or if they are thin, with ral f cc of the whole crystal, and the reiiiaini two may be out w corresponding faces parallel to a natural face, since crystals cut perpendicular to a natural face usually to those of crystals cut parallel to a natural face, although crystals cut similarly somelhe combinations of crystals to be used decoefiicients, whether obtained by the above method of cutting or otherwise, and their reltlie'coeilicients should approach zero.

As a general rule the temperature coefiici nt coeii 'icient of frequency is approximately three times great'as the temperaturecoeficient coeflicient of frequency. 'lhus in Fig. 2 the by three 'rystals having negative temperature coellicicnts and one crystal having a series as in Fi 1. As a rule, this will provide a more precise frequency control than the ar- T his may be explained by reference to Fig". 5. When P1QZO-8lGCtflC crystal is operati:

is suchthat it will operate on a portion of its reactance curve between the points 8-9.

may be OfSllCll frequency that the effective inductanceof the two crystals whose react-- ance curves: are shown, will be as indicated by the ordinates of the points 19 and 11 which are equal and at which points also the slopes of the curves may be equal. T herefore, if a positive temperature coefficient crystal having a reactance curve, such as the curve 12 i Fig. 5, and three negative temperature coeflicient crystals having reactance curves such as the lower curve 13 in Fig. 5, are used, and the temperature coefficients of the negative crystals are one-third as great as the temperature coefficient of the positive crystal, the result will be crystal control of oscillations with zero temperature coeflicient.

The arrangements shown in Figs. 3 and 4.- may be explained by the graphs in Fig.6. Since the slope of the positive coefficient crystal curve is usually greater than that for the negative coeiiicient crystal, agiven change in operating frequency causes thepositive temperature crystal to produce a greater percentage change in inductance than the negative coeiiicient crystal. Since the positive and negative coefficient crystals change their resonant frequencies in about a three to one magnitude ratio with a given temperature change, the inductance contributed by the positive coeflicient crystal should be about one-third that contributed by the negative coeiiicient crystal as regards this factor alone. Combining with the difference in their slopes, the positive coefficient crystal must contribute less than one-third that contributed by the negative coefiicient crystal. The use of the condenser in parallel with the positive coeffici ntcrystal permits adjustment until this condition is reached. Curve 14: represents the reactance curve of the positive coeflicient crystal used and curve 15 represents the reactance curve of the negative coefficient crystal use 1 A condenser connected in shunt to the positive coefficient crystal, as shown in Fig. 3, will afiect the reactance of the crystal indicated by the dotted line 16 in Fig. 6.

p This will change the position of its reactance curve with respect to the frequency of oscillations indicated by the dotted line 17 so as to secure the desired slope relation.

In Fig. 4: a single positive, and a single negative, coeflicient crystal have been used, and these crystals are out to respond at the same frequency. The reactance curve of the positive coefiicient crystal is, however, distorted by the provision of a resistance in shunt thereto in the circuit, so that it has the shape of curve 20 of Fig. 6. This produces a result similar to that produced by the arrangement illustrated in Fig. 3, and curve 16 of Fig. 6. That is, at the frequency at which the oscillator operates the slope of the reactance curve of the negative coefficient crystal as at point 19 is such that its reactance change for a given change in temperature equal to the reactance change of the posltlve temperature coeflicient crystal as at point'QO, for the same change in temperature, and zero temperaturecoeiiicient control is effected. i

The desired relation between the slopes of the reactance curves. of two crystals might also be obtained by cutting the positive cofrequency, say 3 or 4 cycles lower, than the negative coefficient crystal, and operating at a point very near the maximum point of the positive crystal, but on a steeper part of the slope of the negative crystal. I x

It will be understood, of course, thatit is not necessary to use a crystal of any particular shape, as it might be square, oblong, etc., and the arrangement of plates on the crystal may beother thanthat shown. This crystal combination can be placed in any circuit suitable for generating oscillations, and it may be used in any position in any well known circuit in which crystals are customarily employed.

Vhat is claimed is 1 1. A device comprising'an oscillator having. input and output circuits, a plurality of crystal bodies connected in series in one of said circuits, a part of said bodies being adapted to undergo changes of one sign in response to temperature variations and the remainder of saidbodies being adapted to undergo changes similar in character but opposite in sign to the first mentioned changes, in response to temperature variations,whereby temperature effects are compensated.

2. A device comprising an oscillator, a plurality of piezoelectric crystal plates connected in series in the circuitof said oscillator, each of certain of said plates having its thickness in a given direction with respect to a natural crystal axis in the plate and each of certain of said plates having its thickness in another direction with respect to a corresponding natural crystal axis of the plate, whereby temperature effects are compensated.

3. An electrical circuit comprising a plurality of piezoelectric crystal bodies connected in series, certain of said bodies having temperature coefiicients of frequency of one sign, and the remainder of said bodies having temperature coefficients of frequency of opposite sign, whereby temperature effects are compensated. p

4. A circuit in accordance with claim '3, said crystal bodies having substantially the same natural frequency of vibration.

5. An oscillator comprising an electrical space discharge device,-means for causing said device to generate self-sustainedoscillations of a given frequency, and a plurality of piezo-electric crystal bodies connected in series included in said means, said crystal efficient crystal to vibrate at slightly lower bodies having temperature coeficients of frequency opposite in sign, whereby temperature effects are compensated.

6. An oscillator comprising an electric space discharge device having cathode and discharge control elements, means for caussaid device to generate self-sustained 0-scillations of a given frequency, and a piezoelectric crystal device included in said means, said crystal device comprising crystal bodies connected in series between said cathode and discharge control elements, certain of said bodies having temperature coefficients of frequency of one sign and other of said bodies having temperature coefficients of frequency of the opposite sign.

7. An oscillating system comprising an oscillator having input and output circuits, and a plurality of piezo-electric crystals of opposite temperature coen 'icients connected in series in said input circuit for compensating the effect of temperature variations.

8. Au oscillating system comprising an oscillator and a plurality of pierce-electric crystals connected in series, at least one of which has a temperature coefficient of fre quency of opposite sign from the others, whereby temperature effects are compensated.

9. Means for compensating temperature variations in a crystal controlled oscillator comprising two crystals arranged in series in the input circuit of said oscillator, one of said crystals having a positive temperature coefficient of frequency and the other of said crystals having a negative temperature coefficient of frequency.

10. Means f0 producing a constant frecuenc comnrisim a snace discharge oscil- 7 a L: 1 Q

lator having two crystals connected in its input circuit, one of said crystals being cut to vibrate in resonance at a predetermined frequency slightly different from that of the other, whereby temperature effects are compensated.

11. An oscillating system comprising an oscillator and a plurality of piezoelectric crystals connected in the input circuit thereof, one of said crystals being cut to vibrate at a predetermined frequency slightly differ ent from the predetermined frequency at which the balance of said crystals are cutto respond.

12. An oscillating system comprising an oscillator, pair of piezoelectric crystals of opposite temperature coefficients connected in the input circuit thereof, and a condenser connected in shunt to one of said pieZo-electrio crystals, whereby temperature effects are compensated.

13. An oscillating system comprising an oscillator, a piezoelectric crystal having a positive temperature coeflicient of frequency, a piezo-electric crystal having a negative temperature coeflicient of frequency and a condenser connected in shunt to temperature coefficient crystal.

14-. An oscillating system comprising an oscillator, a piezoelectric crystal having a said positive positive temperature coeificient of frequency, and a piezo-electric crystal having a negative temperature coeficient of frequency, said negative temperature coefficient crystal being cut to respond at a frequency a few cycles higher than the frequency at which said positive temperature coefficient crystal responds.

15. An oscillating system comprising an oscillator, a plurality of piezoelectric crystals of opposite temperature coefficient con nected in the input circuit thereof, and a resistance connected in shunt to one of said crystals, whereby temperature effects are compensated.

16. An oscillating system comprising an oscillator, a piezoelectric crystal having a positive temperature coefficient of frequency, a piezoelectric crystal having a negative temperature coefficient of frequency, and a resistance connected in shunt to said positive temperature coetllcient crystal.

In witness whereof, I hereunto subscribe my name this 30th day of April, 1929.

RAYMOND A. HEISING. 

